1. Field of Invention
The present invention relates to a field of nonaqueous electrolytic solutions and a secondary battery using the same. More particularly, this invention pertains to nonaqueous electrolytic solutions that comprise (a) one or more solvents; (b) one or more ionic salts; and (c) one or more additives. The present invention pertains to secondary batteries comprising such nonaqueous electrolytic solutions, and particularly to methods of making nonaqueous electrolytic solutions with a salt additive for use in lithium and lithium ion rechargeable batteries.
2. Description of Related Art
State-of-the-art lithium ion rechargeable (i.e., secondary) batteries commonly use graphite for the anode. In such battery systems, ethylene carbonate (EC) must be used as one of the co-solvents in order to form a stable solid electrolyte interface (SEI) which is beneficial to the cell performance.
However, EC has a high melting point, (ca. 36-39° C.), which limits the performance of lithium and lithium ion batteries in low temperature applications. The addition of a large amount of low viscosity, low melt point co-solvents such as linear carbonates and carboxylate esters can improve cell performance at low temperatures. Unfortunately, such co-solvents have low boiling points and are very flammable, which present problems in high-temperature applications and safety issues.
To that end, propylene carbonate (PC) has been used to fully or partially replace EC to minimize the need for other co-solvents in the electrolytic solutions because PC remains liquid over a wide temperature window (−55° C. to 240° C.). However LiPF6—PC based electrolytic solutions are not compatible with graphite anode in lithium ion rechargeable batteries due to the exfoliation of graphite structure by PC intercalation.
In many cases, certain vinyl compounds such as vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) have been used as additives in electrolytic solutions to help produce the SEI layer. Unfortunately, such vinyl additives can only be used up to about 3% because they decompose at the cathode when present in excess. Further, at relatively high temperatures (e.g., above 50° C.), more components in electrolytic solutions will decompose at the surface of anode material during charging and deposit at the anode. The thickness of the SEI layer increases with time, thus increasing the resistance of the SEI layer and the resistance of charge-transfer process which leads to the increase of total internal resistance of the battery and the battery performance deteriorates. Hence, there is room for improvement in the selection of an electrolyte for use in secondary batteries.